Landmark Case

Execution of the Mentally retarded is "cruel & unusual" punishment under the Eighth Amendment.
Overturned Penry v Lynaugh (1989) . Death penalty is based on
1) Retribution &
2) Deterrence:
MR by definition have diminished capacity and the same factors that make them less morally culpable make it less likely they can understand the possibility of punishment and therefore control their behaviors.